A class-D power amplifier is an electronic amplifier in which the amplifying devices (typically MOSFETs to limit power loss) operate as electronic switches, instead of as linear gain devices as in other amplifiers. The signal received to be amplified is a train of constant amplitude pulses, so the active devices in the amplifier rapidly switch back and forth between a fully conductive (ON) and fully non-conductive (OFF) state.
The analog signal to be amplified is converted to a series of pulses (output pulse train) by pulse width modulation (PWM), pulse density modulation or other similar method before being applied to the amplifier. After amplification, the output pulse train can be converted back to an analog signal by passing the output pulse train through a passive low pass filter (LPF). The major advantage of a class-D amplifier is generally that it can be more efficient than analog amplifiers, with less power being dissipated as heat in the active devices.
Two single-ended channels can be used to create 1 bridge-tied load (BTL) output channel, where a BTL is an output configuration for audio amplifiers that implements a form of impedance bridging. The two channels of a stereo amplifier are fed the same monaural audio signal, with one channel's electrical polarity reversed relative to the other. A loudspeaker is connected between the two amplifier outputs, bridging the output terminals. BTL can double the voltage swing at the load as compared with the same amplifier used without bridging.
Ternary or 1 sinusoidal pulse width (SPW) modulation can reduce idle loss by creating a PWM waveform with a modified duty cycle at idle that is <a 50% duty cycle. The modified duty cycle idle condition reduces inductor ripple current. A low duty cycle (LDC) PWM generally referred to as LDC idle PWM is known to reduce power loss in class-D amplifiers. However, known methods for implementing LDC idle PWM require fully differential loop architectures in closed-loop class D audio amplifiers that need relatively large chip areas because known single-ended configurations result in the audio performance (such as crossover distortion) generally being unacceptable for most user' applications.